Ensure alpha channel in backbuffer is correct with DirectComposition.

ui/gl/gl_surface_stub.cc

Index: ui/gl/gl_surface_stub.cc
diff --git a/ui/gl/gl_surface_stub.cc b/ui/gl/gl_surface_stub.cc
index c49165fee1e5ee857821e4551a8b2764168c1831..ad5b0b3fe4537314cc4bdf28d49132d86376ef03 100644
--- a/ui/gl/gl_surface_stub.cc
+++ b/ui/gl/gl_surface_stub.cc
@@ -9,6 +9,12 @@ namespace gfx {
 void GLSurfaceStub::Destroy() {
 }
 
+bool GLSurfaceStub::Resize(const gfx::Size& size,
+                           float scale_factor,
+                           bool has_alpha) {
+  return true;
+}
+
 bool GLSurfaceStub::IsOffscreen() {
   return false;
 }
@@ -25,6 +31,10 @@ void* GLSurfaceStub::GetHandle() {
   return NULL;
 }
 
+bool GLSurfaceStub::BuffersFlipped() const {
+  return buffers_flipped_;
+}
+
 GLSurfaceStub::~GLSurfaceStub() {}
 
 }  // namespace gfx